AC-to-DC converters which receive power from AC power mains often rectify the sinewave (AC) mains voltage and store energy in a capacitor. The capacitor generally charges to the peak mains voltage such that current only flows into the power supply around the peaks of the input voltage. This causes the waveshape of the input current to the power supply to be a combination of the fundamental mains frequency and integer multiples (harmonics) of the fundamental mains frequency. The magnitudes of these harmonic input currents can be a substantial percentage of the magnitude of the fundamental input current. Accordingly, the harmonic input current can cause distortions which interfere with other susceptible devices connected to the power mains or can cause distortions which accumulate in distribution network elements causing undesirable stress on these elements.
Techniques, often referred to "power factor correction" (PFC) techniques, can be used to reduce the harmonic content of the input current by reforming the input current into what approximates a sinewave. Such power factor circuits are, however, generally complex.
FIG. 1 (Prior Art) is a diagram of one such power factor correction circuit. A boost converter comprising an inductor 1, a high frequency switch 2, a diode 3, a current sense resistor 4, and a control circuit 5 is interposed between the mains terminals 6 and a storage capacitor 7. The control circuitry 5 modulates the conduction of the switch 2 in order to shape the input current into what approximates a sinewave. An isolating DC-to-DC converter comprising a transformer 8, a high frequency switch 9, a control circuit 10, and an output rectifier 11 utilizes the storage capacitor 7 as a voltage source and converts the voltage on the capacitor to a usable DC output voltage level which is supplied onto output terminals 12. The control circuitry 10 of the DC-to-DC converter modulates the conduction of the switch 9 to provide regulation of the output voltage.
Although such prior art power factor correction circuits operate satisfactorily for many applications, the two separate switches 2 and 9 and two separate control circuits 5 and 10 add complexity and cost. Moreover, many applications do not require near total elimination of harmonics but merely require reduction of harmonics below values set by industry standards. A less expensive AC-to-DC converter is therefore sought which draws input currents having reduced harmonics but which does not require two control circuits and two switches.